In the philosophy of mind, a major topic of debate is how the mind - something that is popularly thought to be immaterial - can cause effects in the physical world. That is, how can a person's mental states (i.e. beliefs, desires, etc.) lead to physical events (e.g. raising one's arm)? Beliefs and desires aren't physical things, are they? How much do your beliefs weigh? What is the volume in cubic centimeters of your desires? If mental sates aren't physical, then how can they "cause" physical events? This is commonly referred to as the problem of mental causation.
Dualists (of which there are a great variety) suppose that mind and matter are two ontologically distinct entities. In other words, dualists believe the mind (including its mental states) is real, even though it is immaterial. Physicalists on the other hand deny this, and instead claim that there is no "mind" in the immaterial sense, only the brain. This is because physicalists deny that anything non-physical exists. Thus, it is no surprise that dualists and physicalists offer different solutions to the problem of mental causation.
One obvious way physicalists try to get around the problem is by denying that mental states are non-physical. They would apparently do this by defining a "mental state" as a particular configuration of brain chemistry at a given time. So, my "desire" to drink some water is really nothing more than my neuro-chemistry being in a dehydrated state, which leads to me pouring myself a glass (this is an extreme simplification, but you get the idea). As I see it however, the physicalist position has a major problem; a problem that Inception will help to illustrate.
The goal of the main characters in the film Inception is to implant an idea in a person's mind (don't worry, I'm not giving away the plot), in the hopes that the idea will eventually grow and cause the person to act how they want. It's not exactly brain-washing, but sort of. Through the use of ideas, you're causing a person to think and act a certain way, while making them believe it was their own choice, so to speak. Joseph Gordon-Levitt's character illustrates this concept in the film by telling someone, "Don't think of pink elephants." Inevitably, the person ends up thinking of pink elephants, and the cause of him thinking it was the verbal command not to! Thus, Levitt's character successfully implanted the idea of pink elephants in the man's mind. In my view, this concept provides an entertaining illustration of why the physicalist position on mental causation fails. Here's why:
In order to deny that mental states are immaterial, physicalists appeal to something called the "Completeness Principle." According to this principle, every physical effect must have a sufficient physical cause. In other words, if an effect is physical, then it can always be traced back to an initiating cause that is also physical. The causal chain is "complete" in its physicality. Any appeal to a non-physical cause is superfluous. Physicalists boast that the completeness principle enjoys centuries of empirical support through science. Even so, I think the principle is, well, incomplete.
As Inception illustrated, our communication through language often results in physical effects. The verbal command, "Don't think of pink elephants" causes a person to think of pink elephants. Likewise, the command, "Please pass the salt," results in the physical action of someone raising their arm, grasping the salt, and handing it to you. Let's do a little experiment. If you're reading this right now, raise your right arm over your head and then put it back down again. Did you do it? C'mon, humor me a little. Okay, if you did it, what was ultimately the cause of you raising your arm? If we could trace the chain of causation back, where would it start? Can we agree that the initiating cause of you raising your arm was my suggesting it? If so, was that a physical cause? I don't see how it could be. Can we measure my suggestion? Does it have a weight or volume? No, it does not. If we can "send" ideas through language, and ideas are immaterial, and those ideas cause physical events, then the completeness principle is false. It follows that physicalism is false.
At this point a physicalist may interject and deny that ideas are immaterial things, because they are always in material form. Have you ever experienced an idea separated from matter? Probably not. You hear ideas through sound waves - material particles vibrating at particular frequencies. You see ideas through print - ink on a page, pixels on a screen. You even think about your own ideas with the use of your brain - a labyrinthine amalgam of chemicals, tissue, and electricity. Ideas cannot be separated from matter. The physicalist is right about this. However, it does not follow that ideas and matter are the same thing. The simple fact that the same idea can be communicated through a variety of different material mediums shows that the idea is not the same thing as the matter which expresses it. The ideas that I'm expressing to you right now are not merely pixels on a screen. The ideas are the meaning those pixels happen to represent, not the pixels themselves. We could change the pixels without changing the meaning (ideas). And in the case of one's brain, ideas can actually change the physical medium.
We may summarize the argument thus far. Call it "A Naive Argument Against Physicalism:"
1. If physicalism is true, then the completeness principle is true.
2. The completeness principle is false.
3. Therefore, physicalism is false.
Since this argument is a work in progress, I welcome suggestions, comments, and criticisms.
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1. Read a little about "neuroplasticity." Thinking certain ideas can actually change the physical structure of your brain.